We have been studying the genetic and physical characteristics of sex factor plasmids, F and R, to determine the molecular mechanisms of formation of new genomes or replicons by recombination between two different genome or two different sequences in a genome. We wish to define such phenomena genetic fusion. We have evidence that the particular DNA sequences, including IS sequences, which appear as repeated sequences in plasmids and in the E. coli chromosome, are effectively involved in genetic fusion phenomena. Our future studies will be extensive genetic and physical analysis of such repeated sequences in plasmids and bacteria by using genetic, molecular cloning, and electron microscope heteroduplex techniques. Nucleotide sequencing of IS1 and of the inverted repeat DNA sequences found in a transposon, TnA, may establish a new general concept of genetic fusion at the nucleotide level. We also plan to do physical mapping studies of integration sites and replication origins of an R factor, R100, and the E. coli chromosome. It is aimed at approaching the mechanism of the interesting phenomenon of suppression of replication of the E. coli chromosome by integration of the plasmid. Such a study on "integrative suppression" may be important to the basic understanding of the problem of transformation of eukaryote cells by integration of oncogenic viruses, such as SV40 and Adenovirus.